Inorg. Chem. 2008, 47, 1044−1052
Toward Multifunctional Mo(VI−IV) Complexes: cis-Dioxomolybdenum(VI) Complexes Containing Hydrogen-Bond Acceptors or Donors Lyndal M. R. Hill, Michelle K. Taylor, Victor Wee Lin Ng, and Charles G. Young* School of Chemistry, UniVersity of Melbourne, Victoria 3010, Australia Received October 4, 2007
The complexes cis-TpiPrMoVIO2(OAr-R) (TpiPr ) hydrotris(3-isopropylpyrazol-1-yl)borate, -OAr-R ) hydrogen-bonding phenolate derivative) are formed upon reaction of TpiPrMoO2Cl, HOAr-R, and NEt3 in dichloromethane. The orange, diamagnetic, dioxo−Mo(VI) complexes exhibit strong ν(MoO2) IR bands at ca. 935 and 900 cm-1 and NMR spectra indicative of Cs symmetry. They undergo electrochemically reversible, one-electron reductions at potentials in the range −0.836 to −0.598 V vs SCE; the only exception is the 2-CO2Ph derivative, which exhibits an irreversible reduction at −0.924 V. The complexes display distorted octahedral geometries, with a cis arrangement of terminal oxo ligands and with d(ModO)av ) 1.695 Å and ∠(MoO2)av ) 103.2°. The R groups of the 2-CHO and 2-NHCOMe derivatives are directed away from the oxo groups and into a cleft in the TpiPr ligand; these derivatives are characterized by Mo−O−Cipso angles of ca. 131° (conformation 1). The R group(s) in the 2-CO2Me and 2,3-(OMe)2 derivatives lie above the face of the three O-donor atoms (directed away from the TpiPr ligand) and the complexes display Mo−O−Cipso angles of 153.1(2) and 149.7(2)°, respectively (conformation 2). Conformations 1 and 2 are both observed in the positionally disordered 2-COMe and 2-COEt derivatives, the two conformers having Mo−O− Cipso angles of 130−140 and >150°, respectively. The 3-COMe and 3-NEt2 derivatives have substituents that project away from the TpiPr ligand and Mo−O−Cipso angles of 134.2(2) and 147.7(2)°, respectively. Many of the complexes exhibit fluxional behavior on the NMR time scale, consistent with the rapid interconversion of two conformers in solution.
Introduction Molybdenum enzymes catalyze net oxygen atom transfer reactions involving a wide variety of substrates including carbon monoxide, oxo anions, sulfoxides, N-oxides, aldehydes, purines, and pyrimidines.1-4 They are essential to the health of microorganisms, plants, animals, and humans1-4 and are vital agents in many of the Earth’s biogeochemical cycles.5,6 In broad terms, the mechanisms of action involve a two-electron substrate redox step that interconverts Mo* To whom correspondence should be addressed. E-mail: cgyoung@ unimelb.edu.au. (1) Hille, R. Chem. ReV. 1996, 96, 2757-2816. (2) Pilato, R. S.; Stiefel, E. I. In Bioinorganic Catalysis, 2nd ed.; Reedijk, J., Bouwman, E., Eds.; Marcel Dekker: New York, 1999; pp 81152. (3) Tunney, J. M.; McMaster, J.; Garner, C. D. In ComprehensiVe Coordination Chemistry II; McCleverty, J. A., Meyer, T. J., Eds.; Elsevier Pergamon: Amsterdam, 2004; Vol. 8, Chapter 8.18, pp 459477. (4) Young, C. G. In Encyclopedia of Inorganic Chemistry 2; King, R. B., Ed.; Wiley: Chichester, U.K., 2005; Vol. V, pp 3321-3340. (5) Stiefel, E. I. In Metal Ions in Biological Systems; Sigel, A., Sigel, H., Eds.; Marcel Dekker: New York, 2002; Vol. 39, pp 1-29.
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(VI) and Mo(IV) states.1-4 In some cases, direct oxygen atom transfer (OAT) is invoked, while in others hydroxylation appears to be the key step. There is strong evidence for hydrogen-bond stabilization and proton-transfer activation of key water-based (oxo, hydroxo, and/or aqua) ligands in many of these enzymes.1-4 Substrate redox is followed by regeneration of the active site by sequential, one-electron, coupled electron-proton transfer (CEPT) steps, again involving key water-based ligands.1-4 The combination of OAT and CEPT processes has been achieved in a number of oxoMo model systems.7,8 The effects of hydrogen-bonding9-11 (6) Bertini, I.; Gray, H. B.; Stiefel, E. I.; Valentine, J. S. Biological Inorganic Chemistry: Structure and ReactiVity; University Science Books: Sausalito, CA, 2007. (7) Young, C. G. In Biomimetic Oxidations Catalyzed by Transition Metal Complexes; Meunier, B., Ed.; Imperial College Press: London, 2000; pp 415-459. (8) Young, C. G. In ComprehensiVe Coordination Chemistry II; McCleverty, J. A., Meyer, T. J., Eds.; Elsevier Pergamon: Amsterdam, 2004; Vol. 4, Chapter 4.7, pp 415-527. (9) Ueyama, N.; Okamura, T.; Nakamura, A. J. Am. Chem. Soc. 1992, 114, 8129-8137.
10.1021/ic701957b CCC: $40.75
© 2008 American Chemical Society Published on Web 01/10/2008
cis-Dioxomolybdenum(VI) Complexes
and environment12 on the chemical and redox behavior of oxo-Mo complexes have also attracted considerable attention. In molybdoenzyme model systems featuring hydrotris(pyrazolyl)borate ligands, we have demonstrated forward and reverse OAT reactions involving mononuclear cis-dioxoMo(VI) and oxo-Mo(IV) complexes and CEPT reactions leading to EPR-active oxo(hydroxo)-Mo(V) complexes.7,13,14 Reactions catalyzing the incorporation of water oxygen into oxidized substrates have also been discovered. The formation of Mo(V) species and the incorporation of water oxygen into substrates are proposed to involve intermediate oxo(aqua)Mo(IV) or oxo(hydroxo)-Mo(IV/V) complexes.7,13,14 At the Mo(V) level, cis-dioxo-Mo(V) complexes have been isolated and thoroughly characterized but oxo(hydroxo)-Mo(V) species have only been observed in solution or in combination with the conjugate base.15 Oxo(aqua)-Mo(IV) complexes are rare and few have been isolated and characterized; these invariably adopt a trans geometry, e.g., [MoO(OH2)(CN)4]2- and MoO(OH2)(dppe)2 (dppe ) 1,2bis(diphenylphosphino)ethane).8 The isolation of aqua or hydroxo oxo-Mo(IV,V) complexes may be facilitated by building intramolecular H-bonds into the molecular structure. In tris(pyrazolyl)borate complexes, this can, in principle, be achieved by functionalizing the scorpionate ligand or a coligand. Examples of the first strategy include [CuL(H2O)]PF6 (L ) hydrotris{3-(2-pyridyl)pyrazol-1-yl}borate16 or hydrotris{3-(6-methylpyrid-2yl)pyrazol-1-yl}borate17) where the aqua ligands are H-bonded to pyrazolyl and/or pyridyl nitrogen atoms. Rare earth complexes of the 6-methylpyrid-2-yl ligand also feature aqua ligands stabilized by H-bonds to the pyridyl groups.18 There are many examples of the second strategy in coordination and organometallic chemistry, due to the propensity of aqua ligands to form H-bonds to coligands, counterions, and/or solvent molecules (especially water). Illustrative examples from the recent literature include carboxylate-stabilized M(PMe3)3(O2CR)2(OH2)H2 (M ) Mo, W; R ) Ph, But)19 and methylsquarate-stabilized metal complexes.20 Only a few aqua tris(pyrazolyl)borate complexes are known to participate in intramolecular H-bonding. Where (10) Oku, H.; Ueyama, N.; Nakamura, A. Inorg. Chem. 1997, 36, 15041516 and references cited therein. (11) Conry, R. R.; Tipton, A. A. J. Biol. Inorg. Chem. 2001, 6, 359-366. (12) Basu, P.; Nemykin, V. N.; Sengar, R. S. Inorg. Chem. 2003, 42, 74897501. (13) Xiao, Z.; Bruck, M. A.; Enemark, J. H.; Young, C. G.; Wedd, A. G. Inorg. Chem. 1996, 35, 7508-7515. (14) Laughlin, L. J.; Young, C. G. Inorg. Chem. 1996, 35, 1050-1058. (15) Xiao, Z.; Gable, R. W.; Wedd, A. G.; Young, C. G. J. Am. Chem. Soc. 1996, 118, 2912-2921. (16) Bardwell, D. A.; Jeffery, J. C.; Jones, P. L.; McCleverty, J. A.; Ward, M. D. J. Chem. Soc., Dalton Trans. 1995, 2921-2922. (17) Humphrey, E. R.; Mann, K. L. V.; Reeves, Z. R.; Behrendt, A.; Jeffery, J. C.; Maher, J. P.; McCleverty, J. A.; Ward, M. D. New J. Chem. 1999, 23, 417-423. (18) Reeves, Z. R.; Mann, K. L. V.; Jeffery, J. C.; McCleverty, J. A.; Ward, M. D.; Barigelletti, F.; Armaroli, N. Dalton Trans. 1999, 349-355. (19) Zhu, G.; Parkin, G. Inorg. Chem. 2005, 44, 9637-9639. (20) (a) Alleyne, B. D.; Hosein, H.-A.; Jaggernauth, H.; Hall, L. A.; White, A. J. P.; Williams, D. J. Inorg. Chem. 1999, 38, 2416-24221. (b) Alleyne, B. D.; Williams, A. R.; Hall, L. A.; White, A. J. P.; Williams, D. J. Inorg. Chem. 2001, 40, 1045-1051.
Chart 1. -OAr-R Ligands, Where Numbers Refer to the Corresponding TpiPrMoO2(OAr-R) Complexes
the aqua complexes are charged, H-bonding interactions with the counterions are frequently observed. Interactions of this type are present in the solid-state structure of [Tp*WO(OH2)(MeCCMe)](O3SCF3) (Tp* ) hydrotris(3,5-dimethylpyrazol-1-yl)borate), reported by Crane et al.;21 here, the structural unit is dimeric, the aqua ligands of two cationic complexes being H-bonded to two “bridging” triflate anions. Triflate ions also H-bond to the aqua ligands in a range of hydrotris(3-isopropylpyrazol-1-yl)borate (TpiPr) complexes of Ru, with the general formulas [TpiPrRu(OH2)nL3-n](O3SCF3) (L ) neutral ligand).22 Herein, we report the synthesis and characterization of new dioxo-Mo(VI) complexes, TpiPrMoO2(OAr-R), where -OAr-R represents a phenolate derivative containing potential H-bond acceptors/donors (R). The phenolate coligands included in this study and the numbering scheme for their dioxo-Mo(VI) complexes are given in Chart 1. Related cisdioxo-Mo(VI) complexes of TpiPr have been reported for a variety of simple O- and S-donor coligands.15,23-26 These complexes have served as precursors for novel oxosulfidoMo(VI),27 oxo(phosphine oxide)-Mo(IV),25,28 and carbonyloxo-Mo(IV)26 complexes. Access to multifunctional com(21) Crane, T. W.; White, P. S.; Templeton, J. L. Inorg. Chem. 2000, 39, 1081-1091. (22) Takahashi, Y.; Akita, M.; Hikicki, S.; Moro-oka, Y. Inorg. Chem. 1998, 37, 3186-3194. (23) Xiao, Z.; Bruck, M. A.; Doyle, C.; Enemark, J. H.; Grittini, C.; Gable, R. W.; Wedd, A. G.; Young, C. G. Inorg. Chem. 1995, 34, 59505962. (Erratum: Inorg. Chem. 1996, 35, 5752.) (24) Millar, A. J.; Doonan, C. J.; Laughlin, L. J.; Tiekink, E. R. T.; Young, C. G. Inorg. Chim. Acta 2002, 337, 393-406. (25) Doonan, C. J.; Millar, A. J.; Nielsen, D. J.; Young, C. G. Inorg. Chem. 2005, 44, 4506-4514. (26) Malarek, M. S.; Evans, D. J.; Smith, P. D.; Bleeker, A. R.; White, J. M.; Young, C. G. Inorg. Chem. 2006, 45, 2209-2216. (27) Doonan, C. J.; Nielsen, D. J.; Smith, P. D.; White, J. W.; George, G. N.; Young, C. G. J. Am. Chem. Soc. 2006, 128, 305-316. (28) Millar, A. J.; Doonan, C. J.; Smith, P. D.; Nemykin, V. N.; Basu, P.; Young, C. G. Chem.sEur. J. 2005, 11, 3255-3267.
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Hill et al. plexes lays the groundwork for ongoing investigations aimed at isolating aqua- and hydroxo-Mo(IV,V) complexes relevant to low-valent enzyme states and building extended networks on the basis of H-bonding interactions. Experimental Section Materials and Methods. All reactions were performed under an atmosphere of dinitrogen using dried, deoxygenated solvents, but workups were performed in air. Samples of TpiPrMoO2Cl were prepared as previously described.25 Most of the phenols were obtained from Aldrich Chemical Co. and were generally used without purification; discolored samples were purified by sublimation, so as to assist visual assessment of chromatographic separations. The ethyl salicylate and isopropyl salicylate were prepared from salicylic acid and the appropriate alcohol.29 Column chromatography was performed using silica gel (mesh size 40-200) columns with dimensions of ca. 40-60 cm × ∼2.5 cm diameter. Infrared spectra were recorded on a Bio-Rad FTS 165 FTIR spectrophotometer as pressed KBr disks. Electrospray ionization mass spectrometric (ESI-MS) experiments were carried out in positive-ion mode using a Micromass Quattro II mass spectrometer using samples dissolved in MeCN or MeCN/CH2Cl2 or MeCN/ MeOH mixtures. 1D NMR spectra were recorded at room temperature on a Varian Unity-Plus 400 MHz spectrometer, while COSY, TOCSY, HMBC, and HMQC spectra were recorded on a stabilized Varian Inova-400 MHz spectrometer. Spectra were referenced to residual solvent peaks (for C6D6, δH 7.16, δC 128.39; for CDCl3, δH 7.24, δC 77.0); resonances assigned to the TpiPr ligand are as follows: 5-CH (ring), doublets δ 7.7-7.1 (J ) 2.4 Hz); 4-CH (ring), doublets δ 6.2-5.6 (J ) 2.4 Hz); CH(CH3)2, septets δ 4.9-3.2 (J ) 6.8 Hz); CH(CH3)2, doublets δ 1.3-0.7 (J ) 6.8 Hz); other resonances are due to the -OAr-R groups (note some peaks were broadened (br) or not observed due to fluxionality). Cyclic voltammograms were recorded using a 2 mm glassy carbon working electrode, platinum counter electrode, and a freshly prepared doublejacketed Ag/AgNO3 reference electrode (10 mM AgNO3 in MeCN with 0.1 M NBun4PF6 and clean silver wire), connected to an Autolab Potentiostat operated by the General Purpose Electrochemical System software (version 4.9). Samples were prepared as 1-2 mM solutions in MeCN with 0.1 M NBun4PF6 as supporting electrolyte and scan rates over the range 10-500 mV s-1. Potentials were referenced against the ferrocene couple, Fc+/Fc, and are reported relative to SCE. The Fc+/Fc couple was set to the reported value of +0.400 V vs SCE for acetonitrile/0.1 M NBun4PF6 solutions.30 Microanalyses were performed by Atlantic Microlab Inc., Norcross, GA. Syntheses and Characterization. The procedure below was adopted for all the TpiPrMoO2(OAr-R) complexes; specific conditions or variations are indicated prior to each set of characterization data. Yellow TpiPrMoO2Cl (1.00 g; 2 mmol) and the appropriate phenol (5 mmol) were dissolved in CH2Cl2 (30 mL) in a Schlenk flask, and then triethylamine (3-4 mL, ca. 20-30 mmol) was added. The reaction mixture was stirred vigorously for 1-7 days and then reduced to low volume (ca. 5 mL) by rotary evaporation. The residue was purified by column chromatography on silica gel using the solvents indicated. The complex usually eluted as the third or fourth band (yellow, orange, or red), after unreacted TpiPrMoO2Cl, excess parent phenol, and unidentified minor prod(29) Vogel, A. I.; Furniss, B. S.; Hannaford, A. J.; Smith, P. W. G.; Tatchell, A. R. Vogel’s Textbook of Practical Organic Chemistry, 5th ed.; Longman: London, U.K., 1989; Chapter 6.14, pp 1076-1080. (30) Connelly, N. G.; Gieger, W. E. Chem. ReV. 1996, 96, 877-910.
1046 Inorganic Chemistry, Vol. 47, No. 3, 2008
ucts. The isolated fraction was reduced to dryness and then treated with methanol and refrigerated to yield crystals. The compounds were recrystallized from dichloromethane/hexane or dichloromethane/ methanol mixtures. R ) 2-CHO (1): reaction time 1 day; eluent 1:4 CH2Cl2/hexane; yield 0.50 g (43%). Anal. Calcd for C25H33BMoN6O4: C, 51.00; H, 5.65; N, 14.28. Found: C, 50.55; H, 5.76; N, 14.15. IR (KBr, cm-1): ν(BH) 2519 m and 2470 sh,w; ν(CdO) 1687 s; ν(CN) 1506 m; ν(MoO2) 934 and 903 s. 1H NMR (CDCl3): δH 10.05 (1H, br s), 7.79 (1H, dd, J ) 7.6 and 1.6 Hz), 7.67 (1H, d, J ) 2.4 Hz), 7.64 (2H, d, J ) 2.4 Hz), 7.50 (1H, br m), 7.02 (1H, t, J ) 7.6 Hz), 6.85 (1H, br m), 6.17 (1H, d, J ) 2.4 Hz), 6.09 (2H, d, J ) 2.4 Hz), 4.31 (1H, sept, J ) 6.8 Hz), 3.38 (2H, sept, J ) 6.8 Hz), 1.30 (6H, d, J ) 6.8 Hz), 1.10 (6H, d, J ) 6.8 Hz), 0.85 (6H, d, J ) 6.8 Hz). 13C{1H} NMR (CDCl3): δC 190.7, 167.1, 165.3, 164.8, 137.9, 136.5, 136.0, 127.4, 126.3, 121.6, 120.3, 102.8, 102.3, 28.5, 27.6, 24.5, 23.2, 22.8. ESI-MS: m/z 611.5 [M + Na]+, 589.2 [M + H]+, 479.4 [M - C6H9N2]+. R ) 2-COMe (2): reaction time 3 days; eluent CH2Cl2; yield 0.50 g (42%). Anal. Calcd for C26H35BMoN6O4: C, 51.85; H, 5.86; N, 13.95. Found: C, 51.79; H, 5.94; N, 13.93. IR (KBr, cm-1): ν(BH) 2496 and 2460 m; ν(CdO) 1663 s; ν(CN) 1506 s; ν(MoO2) 935 and 904 s. 1H NMR (C6D6): δH 8.17 (1H, br d, J ) 7.6 Hz), 7.31 (1H, d, J ) 2.4 Hz), 7.14 (2H, d, J ) 2.4 Hz), 7.2-6.6 (3H, br m), 5.78 (2H, d, J ) 2.4 Hz), 5.62 (1H, d, J ) 2.4 Hz), 4.65 (1H, br sept), 3.71 (2H, br sept), ∼2.3 (3H, v br), 1.20 (6H, d, J ) 6.8 Hz), 1.11 (6H, d, J ) 6.8 Hz), 0.85 (6H, br s). 13C{1H} NMR (CDCl3): δC 200.3, 166.1 (br) 165.7, 138.9, 136.8, 131.2, 129.5 (br), 122.3 (br), 103.6, 103.2, 29.4, 28.5, 25.8, 24.1, 23.0 (br). ESI-MS: m/z 627.2 [M + Na]+, 605.2 [M + H]+, 495.2 [M C6H9N2]+. R ) 2-COEt (3): reaction time 3 days; eluent CH2Cl2; yield 0.55 g (45%). Anal. Calcd for C27H37BMoN6O4: C, 52.61; H, 6.05; N, 13.64. Found: C, 52.55; H, 6.05; N, 13.61. IR (KBr, cm-1): ν(BH) 2498 m and 2460 w; ν(CdO) 1668 s; ν(CN) 1506 s; ν(MoO2) 934 and 904 s. 1H NMR (CDCl3): δH 7.74 (1H, d, J ) 8.4 Hz), 7.64 (1H, d, J ) 2.4 Hz), 7.62 (2H, d, J ) 2.4 Hz), ∼7.16.6 (3H, br m), 6.14 (1H, d, J ) 2.4 Hz), 6.04 (2H, br d), 4.30 (1H, br), 3.34 (2H, br), ∼2.0 (2H, br m) 1.27 (6H, d, J ) 6.8 Hz), 1.07 (6H, d, J ) 6.8 Hz), 0.85 (v br), 0.56 (v br). 1H NMR (C6D6): δH 8.13 (br s), 7.32 (d, J ) 2.4 Hz), 7.14 (d, J ) 2.4 Hz), 6.71 (br s), 5.79 (d, J ) 2.4 Hz), 5.62 (d, J ) 2.4 Hz), 4.65 (br s), 3.72 (br s), ∼2.3 (br), 1.19 (d, J ) 6.8 Hz), 1.12 (d, J ) 6.4 Hz), 0.85 (br s). 13C{1H} NMR (CDCl3): δC 202.3 (br), 165.0 (br), 164.6, 137.8, 135.6, 134-5 (br), 130.1, 128.5, 121.3, 119-120 (br), 102.4, 102.1, 36-38 (br), 28.3, 27.4, 24.7, 23.0 (2C), 21.8. ESI-MS: m/z 641.2 [M + Na]+, 619.3 [M + H]+, 509.2 [M - C6H9N2]+. R ) 2-CO2Me (4): reaction time 4 days; eluent CH2Cl2; yield 0.40 g (32%). Anal. Calcd for C26H35BMoN6O5: C, 50.47; H, 5.71; N, 13.59. Found: C, 50.16; H, 5.71; N, 13.47. IR (KBr, cm-1): ν(BH) 2485 and 2458 m; ν(CdO) 1701 s; ν(CN) 1507 s; ν(MoO2) 933 and 905 s. 1H NMR (CDCl3): δH 7.80 (2H, dd, J ) 7.8 and 1.6 Hz), 7.62 (1H, d, J ) 2.4 Hz), 7.60 (2H, d, J ) 2.4 Hz), 6.90 (2H, br m), 6.15 (1H, d, J ) 2.4 Hz), 6.06 (2H, d, J ) 2.4 Hz), 4.40 (1H, br sept, J ) 6.8 Hz), 3.43 (2H, br sept), 1.29 (6H, d, J ) 6.8 Hz), 1.08 (6H, d, J ) 6.8 Hz), 0.77 (6H, br s) (COCH3 resonance not observed). 13C{1H} NMR (CDCl3): δC 167.5, 165.1, 164.8, 137.5, 135.4, 134.0 (br), 131.7, 121.1 (br), 120.1 (br), 102.5, 102.0, 51.7, 28.4, 27.4, 24.8 23.3, 22.2. ESI-MS: m/z 641.3 [M + Na]+, 619.4 [M + H]+, 509.5 [M -C6H9N2]+. R ) 2-CO2Et (5): reaction time 1 day; eluent 1:1 CH2Cl2/ hexane; yield 0.41 g (32%). Anal. Calcd for C27H37BMoN6O5: C, 51.28; H, 5.90; N, 13.29. Found: C, 51.04; H, 6.03; N, 13.33. IR
cis-Dioxomolybdenum(VI) Complexes (KBr, cm-1): ν(BH) 2489 m and 2459 sh, w; ν(CdO) 1697 s; ν(CN) 1508 s; ν(MoO2) 933 and 905 s. 1H NMR (CDCl3): δH 7.87 (1H, dd, J ) 6.8 and 1.6 Hz), 7.64 (1H, d, J ) 2.4 Hz), 7.62 (2H, d, J ) 2.4 Hz), 6.90 (2H, br m), 6.15 (1H, d, J ) 2.4 Hz), 6.06 (2H, d, J ) 2.4 Hz), 4.41 (1H, sept, J ) 6.8 Hz), 3.44 (2H, br sept), 1.30 (6H, d, J ) 6.8 Hz), 1.10 (6H, d, J ) 6.8 Hz), 0.83 (6H, br s) (one Ar-H and COCH2CH3 resonances not observed). 13C{1H} NMR (CDCl ): δ 167.4, 165.2, 165.0, 137.5, 135.8, 3 C 135.5, 132.0 (br), 121.0 (br), 102.6, 102.0, 60.8 (br), 28.4, 27.4, 24.7, 23.3, 22.3. ESI-MS: m/z 657.3 [M + Na]+, 635.2 [M + H]+, 525.3 [M - C6H9N2]+. R ) 2-CO2iPr (6): reaction time 1 day; eluent 1:1 CH2Cl2/ hexane; yield 0.69 g (54%). Anal. Calcd for C28H39BMoN6O5: C, 51.99; H, 6.08; N, 13.00. Found: C, 52.17; H, 6.14; N, 13.04. IR (KBr, cm-1): ν(BH) 2489 m and 2459 sh, m; ν(CdO) 1694 s; ν(CN) 1507 s; ν(MoO2) 933 and 904 s. 1H NMR (CDCl3): δH 7.80 (br), 7.64 (1H, d, J ) 2.4 Hz), 7.62 (2H, d, J ) 2.4 Hz), 6.90 (br), 6.15 (1H, d, J ) 2.4 Hz), 6.06 (2H, d, J ) 2.4 Hz), 5.0 (vbr), 4.4 (br), 3.4 (br), 1.29 (3H, d, J ) 6.8 Hz), 1.09 (3H, d, J ) 6.8 Hz), 0.6-1.0 (br). 13C{1H} NMR: spectrum could not be obtained due to decomposition during data acquisition. ESI-MS: m/z 671.3 [M + Na]+, 649.3 [M + H]+. R ) 2-CO2Ph (7): reaction time 3 days; eluent 1:1 CH2Cl2/ hexane; yield 0.35 g (26%). Anal. Calcd for C31H37BMoN6O5: C, 54.69; H, 5.48; N, 12.35. Found: C, 53.94; H, 5.49; N, 12.20. IR (KBr, cm-1): ν(BH) 2506 m and 2460 sh, w; ν(CdO) 1774 s; ν(CN) 1508 s; ν(MoO2) 925 and 900 s. 1H NMR (CDCl3): δH 7.95 (1H, dd, J ) 7.8 and 2.0 Hz), 7.64 (1H, d, J ) 2.4 Hz), 7.57.1 (10H, br m), 6.15 (1H, d, J ) 2.4 Hz), 5.97 (2H, br d), 4.37 (1H, sept, J ) 6.8 Hz), 3.46 (2H, br sept), 1.29 (6H, d, J ) 6.8 Hz), 1.10 (6H, d, J ) 6.8 Hz), 0.93 (6H, br s). 13C{1H} NMR (CDCl3): δC 165.6, 165.1, 164.8, 137.6, 135.4, 132.2, 129.0 (br), 125.2 (br), 121.5 (br), 121.2 (br), 120.7 (br), 102.5, 102.1, 28.4, 27.5, 24.8, 23.3, 22.3. ESI-MS: m/z 603.6 [M + Na]+, 681.2 [M + H]+, 571.6 [M - C6H9N2]+. R ) 2-CONHPh (8): reaction time 1 day; eluent CH2Cl2; yield 0.87 g (64%). Anal. Calcd for C31H38BMoN7O4: C, 54.80; H, 5.64; N, 14.43. Found: C, 54.09; H, 5.71; N, 14.17. IR (KBr, cm-1): ν(NH) 3315 s; ν(BH) 2521 sh, w and 2484 m; ν(CdO) 1659 s; ν(CN) 1504 s; ν(MoO2) 935 and 904 s. 1H NMR (CDCl3): δH 9.93 (1H, br s), 8.33 (1H, d, J ) 8.0 Hz), 7.74 (1H, d, J ) 2.4 Hz), 7.64 (3H, br m), 7.50 (1H, d, J ) 8.0 Hz), 7.19 (1H, t, J ) 7.8 Hz), 6.96 (2H, t, J ) 7.6 Hz), 6.88 (1H, d, J ) 7.6 Hz), 6.33 (2H, d, J ) 7.6 Hz), 6.22 (1H, d, J ) 2.4 Hz), 5.98 (2H, d, J ) 2.4 Hz), 4.25 (1H, sept, J ) 6.8 Hz), 3.26 (2H, sept, J ) 6.8 Hz), 1.31 (6H, d, J ) 6.8 Hz), 1.07 (6H, d, J ) 6.8 Hz), 0.91 (6H, d, J ) 6.8 Hz). 13C{1H} NMR (CDCl ): δ 165.7, 163.2, 162.7, 138.3, 138.2, 3 C 136.3, 134.2, 131.7, 128.4, 123.3, 122.3, 122.1, 121.0, 119.2, 103.1, 103.0, 28.6, 27.6, 24.7, 23.2, 22.3. ESI-MS: m/z 682.2 [M + H]+, 572.4 [M - C6H9N2]+. R ) 2-OMe (9): reaction time 3 days; eluent CH2Cl2; yield 0.95 g (80%). Anal. Calcd for C25H35BMoN6O4: C, 50.80; H, 5.98; N, 14.24. Found: C, 50.71; H, 6.02; N, 14.12. IR (KBr, cm-1): ν(BH) 2520 m and 2466 w; ν(CN) 1507 s; ν(MoO2) 926 and 901 s. 1H NMR (CDCl ): δ 7.58 (1H, d, J ) 2.4 Hz), 7.56 (1H, d, J ) 3 H 2.4 Hz), 6.85-6.74 (2H, m), 6.70 (1H, dt, J ) 8.0 and 2.0 Hz), 6.47 (1H, d, J ) 7.6 Hz), 6.09 (1H, d, J ) 2.4 Hz), 6.01 (2H, d, J ) 2.4 Hz), 4.4 (1H, sept, J ) 6.8 Hz), 3.65 (3H, s), 3.43 (2H, sept, J ) 6.8 Hz), 1.24 (6H, d, J ) 6.8 Hz), 1.06 (6H, d, J ) 6.8 Hz), 0.82 (6H, d, J ) 6.8 Hz). 13C{1H} NMR (CDCl3): δC 165.9, 165.7, 154.2, 151.1, 138.0, 136.4, 122.7, 122.0, 120.4, 113.1, 103.3, 102.7, 56.7, 29.3, 28.3, 25.3, 24.3, 23.4. ESI-MS: m/z 593.6 [M + H]+, 483.5 [M - C6H9N2]+.
R ) 2,3-(OMe)2 (10): reaction time 3 days; eluent CH2Cl2; yield 1.10 g (90%). Anal. Calcd for C26H37BMoN6O5: C, 50.34; H, 6.01; N, 13.55. Found: C, 49.79; H, 5.85; N, 13.16. IR (KBr, cm-1): ν(BH) 2503 m and 2466 w; ν(CN) 1507 s; ν(MoO2) 925 and 903 s. 1H NMR (C6D6): δH 7.36 (2H, d, J ) 2.4 Hz), 7.19 (1H, d, J ) 2.4 Hz), 6.61 (1H, t, J ) 8.2 Hz), 6.28 and 6.24 (each 1H, dd, J ) 8.2 and 1.6 Hz), 5.87 (2H, d, J ) 2.4 Hz), 5.66 (1H, d, J ) 2.4 Hz), 4.83 (1H, sept, J ) 6.8 Hz), 3.98 (2H, sept, J ) 6.8 Hz), 3.78 (3H, s), 3.33 (3H, s), 1.23 (6H, d, J ) 6.8 Hz), 1.20 (6H, d, J ) 6.8 Hz), 1.00 (6H, d, J ) 6.8 Hz). 13C{1H} NMR (C6D6): δC 165.7, 165.7, 158.1, 154.8, 140.9, 137.6, 135.9, 123.4, 113.4, 107.0, 103.1, 102.7, 61.1, 56.1, 29.0, 28.2, 25.2, 23.6, 23.0. See Supporting Information for complete assignments of 1H and 13C NMR spectra. ESI-MS: m/z 645.2 [M + Na]+, 623.2 [M + H]+, 513.2 [M C6H9N2]+. R ) 3-COMe (11): reaction time 3 days; eluent CH2Cl2; yield 0.55 g (46%). Anal. Calcd for C26H35BMoN6O4: C, 51.85; H, 5.86; N, 13.95. Found: C, 51.50; H, 5.91; N, 13.79. IR (KBr, cm-1): ν(BH) 2486 and 2460 m; ν(CdO) 1683 s; ν(CN) 1507 s; ν(MoO2) 931s and 900 s. 1H NMR (CDCl3): δH 7.62 (1H, br d), 7.60 (2H, br d), 7.48 (1H, d, J ) 7.2 Hz), 7.28 (1H, t, J ) 7.2 Hz), 6.99 (1H, m), 6.12 (1H, br d), 6.04 (2H, br d), 4.32 (1H, sept, J ) 6.8 Hz), 3.42 (2H, sept, J ) 6.8 Hz), 2.39 (3H, s), 1.25 (6H, d, J ) 6.8 Hz), 1.07 (6H, d, J ) 6.8 Hz), 0.81 (6H, d, J ) 6.8 Hz). 13C{1H} NMR (CDCl3): δC (CO resonance not observed): 166.2, 165.6, 164.5, 139.2, 138.5, 136.6, 130.5, 124.7, 121.9, 119.8, 103.5, 103.0, 29.3, 28.4, 27.8, 25.2, 24.2, 23.8. ESI-MS: m/z 605.2 [M + H]+. R ) 2-NHCOMe (12): reaction time 5 days; eluent 9:1 CH2Cl2/ethyl acetate; yield 0.49 g (40%). Anal. Calcd for C26H36BMoN7O4: C, 50.58; H, 5.88; N, 15.88. Found: C, 50.66; H, 5.90; N, 15.89. IR (KBr, cm-1): ν(NH) 3401 s; ν(BH) 2498 m, 2462 m; ν(CdO) 1690 s; ν(CN) 1506 s; ν(MoO2) 935 and 903 s. 1H NMR (CDCl3): δH 8.21 (1H, apparent d), 7.62 (3H, overlapping d), 7.36.9 (4H, br m), 6.13 (1H, d, J ) 2.4 Hz), 6.09 (2H, br d), 4.27 (1H, sept, J ) 6.8 Hz), 3.31 (2H, sept, J ) 6.8 Hz); 1.25 (6H, d, J ) 6.8 Hz), 1.05 (6H, d, J ) 6.8 Hz), 0.81 (6H, br s). 13C{1H} NMR (CDCl3): δC 165.2, 165.1, 137.7, 135.8, 124.0 (br), 122.3 (br), 119.2 (br), 117.0, 102.6, 102.5, 102.3, 28.3, 27.4, 24.6, 23.0, 22.2 (br). ESI-MS: m/ z 642.2 [M + Na]+, 620.3 [M + H]+, 510.2 [M - C6H9N2]+. R ) 2-NH2-5-Me (13): reaction time 5 days; eluent CH2Cl2; yield
